1. Field of the Invention
The present invention relates to a voltage output device for an electronic system, and more particularly to a voltage output device for an electronic system for preventing components from damage by a huge current.
2. Description of the Prior Art
Analog pulse modulation, widely used in the industry, transforms sampling values of a message into features of amplitudes, periods or positions, and applies a sequence of periodical pulses as a carrier, which carries the features with each pulse sequentially. Therefore, according to different formats of the features, the prior art pulse modulation can be classified into pulse amplitude modulation, pulse width modulation, and pulse frequency modulation.
Since pulse modulation can continuously present variation of the sampling values, pulse modulation is widely used in a system having feedback outputs, such as a power supply device, a voltage converter or a driving device, etc., for providing a stable output signal. For example, please refer to FIG. 1. FIG. 1 is a schematic diagram of a voltage output device 10 according to the prior art. The voltage output device 10 is utilized for outputting a specific output voltage VOUT to a load 110 according to an input voltage VIN. The voltage output device 10 comprises a transistor 100, a transistor 102, a diode 104, an inductor 106, a capacitor 108, and a pulse control unit 112. The pulse control unit 112 is designed according to operations of pulse amplitude modulation, pulse width modulation or pulse frequency modulation. The pulse control unit 112 can control switch timings of the transistor 100 and the transistor 102 according to a specific control signal, an input voltage V1 of the inductor 106 or an output voltage of the inductor 106 (related connection wires are not shown in FIG. 1), so as to adjust the output voltage VOUT of the inductor 106 to make the output voltage VOUT to conform to some conditions, such as a specific multiple of the input voltage VIN, a low-pass-filtering result of the input voltage VIN, and so on.
Therefore, the pulse control unit 112 can transform the input voltage VIN into the specific output voltage VOUT through controlling the switch timings of the transistor 100 and the transistor 102. In some situations, such as to enhance driving capability or be applied in a high power application, the transistor 100 is implemented by an n-type metal-oxide-semiconductor transistor. Since the operation condition of an n-type metal-oxide-semiconductor transistor is a voltage drop between a gate and a source greater than a threshold voltage, a boost circuit is needed in the voltage output device 10 for providing a sufficient bias voltage for the transistor 100 and the transistor 102. Please refer to FIG. 2. FIG. 2 is a schematic diagram of a voltage output device 20 according to the prior art. The voltage output device 20 is the voltage output device 10 plus a boost circuit 200, and driving units 206 and 208. The boost circuit 200 comprises a diode 202 and a capacitor 204, and is coupled between a voltage source VCC and a first node, which is a joint of the transistor 100, the transistor 102, the diode 104 and the inductor 106. A power input terminal of the driving unit 206 is coupled between the diode 202 and the capacitor 204, and a grounding terminal of the driving unit 206 is couple to the first node. A power input terminal of the driving unit 208 is coupled to the voltage source VCC, and a grounding terminal of the driving unit 208 is couple to the ground. In addition, in FIG. 2, V2, V3, and V4 respectively present a gate voltage of the transistor 100, a gate voltage of the transistor 102, and an output voltage of the diode 202.
Please continue referring to FIG. 3. FIG. 3 is a schematic diagram of signals related to the voltage output device 20 shown in FIG. 2. In the beginning, the voltage V2 is LOW and the voltage V3 is HIGH, so the transistor 102 is turned on and the voltage V1 is in a ground level. As a result, the diode 202 keep charging to the capacitor 204 until V4=VCC−0.7V (0.7V is cut-in voltage of the diode 202.) Next, the voltage V2 changes to HIGH and the voltage V3 changes to LOW, so the transistor 100 is turned on and the transistor 102 is cut off. As a result, the voltage V1 approaches to the voltage VIN, and the voltage V4 is pulled up to VIN+VCC−0.7V. Moreover, in order to prevent the transistor 100 and the transistor 102 from conducting at the same time, which results in damage of components, there is a default time interval between changing states of the voltage V2 and the voltage V3, for simultaneously turning off the transistor 100 and the transistor 102 for a while. In such a case, when the operating status of the voltage output device 20 is between the voltage V3 changing to LOW and the voltage V2 changing to HIGH, since the load 110 keeps sinking current and the inductor 106 keeps current from changing, the inductor 106 turns on the diode 104 to sink current from the ground, and the voltage V1 changes from GND to −0.7V. At the moment, the diode 202 keeps charging to the capacitor 204 until the voltage V4=VCC−V1−0.7V=VCC−1.4V. Afterwards, the voltage V2 changes to HIGH, then the transistor 100 is turned on, and the diode 202 will sink a huge current to reverse the PN-junction. At this time, if the diode 202 cannot bear such a high power, the diode 202 will be damaged.
In a word, in the prior art, the diode in the boost circuit is required to bear a huge power, which increases production cost.